1. Field of the Invention
The present invention relates to a system and method for dewatering and producing gas from coalbed and shale seams utilizing underbalanced multilateral drilling techniques.
2. General Background of the Invention
In the drilling of wells, one of the most critical elements in drilling has always been to maintain the well in a hydrostatically balanced state, so that should the drill bit strike a pocket of hydrocarbons, that the formation pressure does not overcome the hydrostatic pressure of the drill fluid column in the well, and thus a blow out does not occur. In conventional drilling, what has always been done, is during the drilling process, to flow heavy fluids; i.e., muds, into the drill bore during drilling, so that the hydrostatic pressure of the muds within the borehole is heavier than the pressure from the formation. Therefore, any potential blowout which may occur otherwise is prevented due to the heavy muds which create the higher hydrostatic pressure downward into the formation.
It has been recently found, that when such a hydrostatic head is placed on the formation, often times the heavy muds or fluids flow into the formation, and by doing so, create severe damage of the formation, which is a detriment to the productivity of the formation. Therefore, there has been developed the technique that is called underbalanced drilling, which technique allows for greater production, and does not create formation damage which would impede the production process. Furthermore, it has been shown that productivity is enhanced in multilateral wells combined with the non-formation damaging affects of the underbalanced drilling. These results are accomplished by introducing a lighter fluid such as nitrogen or air into the drill hole, or a combination of same or other type fluids or gases, sufficiently as to create an underbalance so that fluid in the borehole does not move into the formation during drilling. In order to accomplish this, often times the drilling is undertaken through the use of coil tubing, or jointed pipe systems in conjunction with aerated fluids. Another technique of underbalanced drilling is referred to as micro-annulus drilling where a low pressure reservoir is drilled. In effect, a string of casing is lowered into the well bore and utilizing a two string drilling technique, there is circulated a lighter fluid down the outer annulus, which lowers the hydrostatic pressure of the fluid column, thus relieving the formation. This allows the fluid column to be lighter than the formation pressure which, if it weren't, would cause invasion of drilling fluid and solids to enter the formation which is detrimental to productivity. By utilizing this system, drillers are able to circulate a lighter fluid which can return up either inner or outer annulus, which enables them to circulate with a different fluid down the drill string. In doing so, basically air and nitrogen are being introduced down the system which allows them to circulate two different combination fluids with two different strings.
The technology utilized in underbalanced drilling of oil and gas wells can also be applied to the process of de-watering and disposal of produced water when drilling to recover coalbed methane and shale gas. There exists an estimated total more than 700 trillion cubic feet of coalbed methane gas accumulations in the United States and some 7500 trillion cubic feet worldwide. The use of underbalanced drilling techniques is a very efficient, cost effective manner of recovering this huge methane gas resource. The underbalanced techniques heretofore utilized for oil and gas recovery, as disclosed and claimed in U.S. Pat. No. 5,720,350 and No. 6,045,550, both by the same inventor, and incorporated herein by reference thereto.